diff --git a/backend/packages/harness/deerflow/tools/builtins/list_uploaded_files_tool.py b/backend/packages/harness/deerflow/tools/builtins/list_uploaded_files_tool.py
index 142675b49..c6212fc4d 100644
--- a/backend/packages/harness/deerflow/tools/builtins/list_uploaded_files_tool.py
+++ b/backend/packages/harness/deerflow/tools/builtins/list_uploaded_files_tool.py
@@ -12,7 +12,7 @@ from collections import Counter
 from pathlib import Path
 from typing import Annotated, Any
 
-from langchain.tools import InjectedToolArg, tool
+from langchain.tools import tool
 from langgraph.config import get_config
 
 from deerflow.agents.middlewares.input_sanitization_middleware import neutralize_untrusted_tags
@@ -187,6 +187,7 @@ def _list_uploaded_files_impl(
 
 @tool
 def list_uploaded_files(
+    runtime: Runtime,
     include_outline: Annotated[
         bool | list[str],
         "Control which files get their document outline (headings/preview) returned. "
@@ -198,7 +199,6 @@ def list_uploaded_files(
         int,
         "Maximum number of files to return (default 20, max 100).",
     ] = _DEFAULT_MAX_RESULTS,
-    runtime: Annotated[Runtime, InjectedToolArg] | None = None,
 ) -> dict:
     """Discover historical uploaded files available in this thread.
 
diff --git a/backend/tests/test_list_uploaded_files_tool.py b/backend/tests/test_list_uploaded_files_tool.py
index 5b68edaa6..4987e98b2 100644
--- a/backend/tests/test_list_uploaded_files_tool.py
+++ b/backend/tests/test_list_uploaded_files_tool.py
@@ -840,3 +840,33 @@ def test_all_string_fields_in_result_are_neutralized(tmp_path):
     # Sanity: the result is non-empty and well-formed
     assert len(result["files"]) == 1
     assert result["total_count"] == 1
+
+
+# ---------------------------------------------------------------------------
+# @tool schema — regression for #4375
+# ---------------------------------------------------------------------------
+class TestToolSchema:
+    """Guard the model-facing schema of the @tool wrapper.
+
+    The injected ``runtime`` argument must not leak into the schema sent to the
+    LLM. Declaring it as ``Annotated[Runtime, InjectedToolArg] | None`` (issue
+    #4375) made the top-level annotation a Union, so LangChain no longer treated
+    it as injected and pydantic raised ``PydanticInvalidForJsonSchema`` on the
+    ToolRuntime dataclass the moment the tool was bound to a model.
+    """
+
+    def test_runtime_excluded_from_model_facing_args(self):
+        from deerflow.tools.builtins.list_uploaded_files_tool import list_uploaded_files
+
+        assert set(list_uploaded_files.args) == {"include_outline", "max_results"}
+        assert "runtime" not in list_uploaded_files.args
+
+    def test_openai_schema_generation_succeeds(self):
+        from langchain_core.utils.function_calling import convert_to_openai_tool
+
+        from deerflow.tools.builtins.list_uploaded_files_tool import list_uploaded_files
+
+        # This raised PydanticInvalidForJsonSchema before the fix.
+        oai = convert_to_openai_tool(list_uploaded_files)
+        params = oai["function"]["parameters"]["properties"]
+        assert set(params) == {"include_outline", "max_results"}